Several members of the protein kinase family have been clearly implicated in the pathogenesis of various proliferative diseases and thus represent important targets for treatment of these diseases. Some of the proliferative diseases relevant to this invention include cancer, rheumatoid arthritis, atherosclerosis, and retinopathies. Important examples of kinases which have been shown to cause or contribute to the pathogenesis of these diseases including, but not limited to, are BRaf, CRaf, Abl, KDR(VEGF), EGFR/HER1, HER2, HER3, cMET, FLT-3, PDGFR-a, PDGFR-b, p38, cKIT, and JAK2.
A major signaling pathway downstream of cell surface growth factor receptor activation is the Ras-RAF-MEK-ERK-MAP kinase pathway (Peyssonnaux, C. et al, Biol. Cell (2001) 93: 53-62, Cancers arise when mutations occur in one or more of the proteins involved in this signaling cascade. Cell proliferation and differentiation become dysregulated and cell survival mechanisms are activated which allow unregulated cancer cells to override protective programmed cell death surveillance. Mutations in the p21-Ras protein have been shown to be a major cause of dysregulation of this signaling pathway, leading to the development of human cancers. P21-Ras mutations have been identified in approximately 30% of human cancers (Bolton et al, Ann. Rep. Med. Chem. (1994) 29: 165-174). Cancer-causing mutations in the P21-Ras protein lead to a constitutively active signaling cascade, causing unregulated activation of the downstream components of the RAF-MEK-ERK-MAP kinase pathway (Magnuson et al., Semin. Cancer Biol. (1994) 5: 247-253). The three RAF kinases which participate in this signaling cascade are known as ARAF, BRAF, and CRAF (Peyssonnaux, C. et al, Biol. Cell (2001) 93: 53-62; Avruch, J., Recent Prog. Horm. Res. (2001) 56: 127-155; Kolch, W., Biochem. J. (2000) 351: 289-305). These RAF kinase isoforms are all activated by Ras, and thus are activated in cancers that result from mutated and upregulated p21-Ras protein activity. In addition to activation of this signaling cascade at the initial p21-Ras protein level, mutations have also been found in BRAF kinase which results in activation of the cascade downstream from p21-Ras (Davies, H., et al, Nature (2002) 417: 949-954). A dominant single site mutation at position 600 in the BRAF kinase was shown to be particularly aggressive and linked to approximately 80% of the observed human malignant melanomas. This mutation substitutes the negatively charged amino acid glutamic acid for the normally occurring neutral amino acid valine. This single site mutation is sufficient to render the mutated BRAF kinase constitutively active, resulting in signaling pathway dysregulation and human cancer. Hence small molecule inhibitors of BRAF kinase are a rational approach to the treatment of human malignancy, whether the signaling mutation is at the level of the upstream p21-Ras protein or at the level of BRAF kinase.
The majority of small molecule kinase inhibitors that have been reported have been shown to bind in one of three ways. Most of the reported inhibitors interact with the ATP binding domain of the active site and exert their effects by competing with ATP for occupancy. Other inhibitors have been shown to bind to a separate hydrophobic region of the protein known as the “DFG-in-conformation” pocket, and still others have been shown to bind to both the ATP domain and the “DFG-in-conformation” pocket. Examples specific to inhibitors of RAF kinases can be found in Lowinger et al, Current Pharmaceutical Design (2002) 8: 2269-2278; Dumas, J. et al., Current Opinion in Drug Discovery & Development (2004) 7: 600-616; Dumas, J. et al, WO 2003068223 A1 (2003); Dumas, J., et al, WO 9932455 A1 (1999), and Wan, P. T. C., et al, Cell (2004)116: 855-867.
Physiologically, kinases are regulated by a common activation/deactivation mechanism wherein a specific activation loop sequence of the kinase protein binds into a specific pocket on the same protein which is referred to as the switch control pocket (see Flynn, D. et al WO 2004/061084 for further details). Such binding occurs when specific amino acid residues of the activation loop are modified for example by phosphorylation, oxidation, or nitrosylation. The binding of the activation loop into the switch pocket results in a conformational change of the protein into its active form (Huse, M. and Kuriyan, J. Cell (109) 275-282.)